The following published patent documents are believed to represent the current state of the art:
U.S. Pat. Nos. 6,315,796 and 6,605,116;
French Patent Nos. 2,859,098 and 2,862,523; and
U.S. Patent Application Publication Nos. 2001/0010024; 2003/0074084 and 2004/0148024.